As the smallest of the terrestrial planets of our solar system, Mercury has a lot to teach us about how small rocky planets evolve. Unlike Earth, whose outer strong layer—the lithosphere—is broken into a mosaic of plates that shift relative to one another, Mercury is a one-plate planet. This view of Mercury has been determined from images and data returned by two spacecraft, Mariner 10 and MESSENGER. On Earth, the motion of the plates is driven by the flow of hot, semi-molten rock in the mantle—the layer the lithosphere literally floats on. Flow of this semi-molten rock is a mechanism by which the Earth cools its interior. There is abundant evidence of this mantle driven process at Earth’s plate margins where large faults and volcanic activity are concentrated.
On a one-plate planet, heat loss is manifested differently. Instead of movement and interaction of multiple plates, the loss of interior heat acts on the single plate, causing it to contract and shrink. The evidence Mercury has contracted is in the form of a population of globally distributed, mountainous fault scarps. As the planet’s interior cools, the crust—the rocky, upper part of the lithosphere—is forced to contract, forming thrust fault scarps hundreds of kilometers long and many with over a kilometer or more of relief. However, forces from global contraction alone should result in a global array of faults that are uniformly scattered around the planet.
Before the MESSENGER spacecraft’s flybys and orbit of Mercury, I studied the fault scarps detected in images from the flybys of Mariner 10. Although the full extent of Mercury’s fault scarp population wasn’t known, there were indications that some faults were grouped into long, linear clusters. MESSENGER confirmed the existence of long clusters, or belts, of thrust fault scarps, some extending over thousands of kilometers. Because these clusters are very likely not the result of global contraction, some other process is causing faults to form in linear belts. Could it be that something going on in Mercury’s mantle is responsible?
My colleagues and I have introduced new models of the thickness of Mercury’s crust, created using gravity and topographic data obtained by MESSENGER. In our analysis, we find that the clusters of fault scarps are found in areas of thick crust. The association between the clusters and the thickest crust may be evidence of flow in Mercury’s mantle. We suggest that downward mantle flow could thicken and contract Mercury’s crust, helping to form the long belts of fault scarps. We also introduce a new model of mantle dynamic pressure that shows positive values, indicating upward flow, and negative values, indicating downward flow. This model shows a correlation between regions with greater numbers of fault scarps, or higher contractional strain, and regions of negative mantle dynamic pressure.
Downward mantle flow is a process that has been proposed for intra-plate tectonics on Earth where thrust faults form far from plate margins. A connection between mantle flow and tectonics suggests a similar process like that on Earth has influenced the formation of large faults on Mercury, and perhaps still is.